The invention relates generally to optical communications, and relates more particularly to modulators used in optical communications links.
In any optical communication link, there are three major components: a light source that generates light, a modulator that encodes the light, and a photodiode that detects the light. Silicon has recently been considered as a material for use in modulators for optical communications links; however, to date, such silicon modulators have been characterized by drawbacks such as large device footprints and/or limited modulation bandwidth and optical bandwidth.
FIG. 1 is a cross-sectional view illustrating a portion of a conventional metal-oxide-semiconductor (MOS) capacitor 100. Specifically, the capacitor 100 comprises a first silicon layer 102 and a second silicon layer 108 having a gate oxide layer 104 disposed therebetween. A phase shift is induced in the optical mode of light propagating through the capacitor 100 when the refractive index of the silicon is modified, e.g., due to a surface charge produced by applied voltage (manifested in carriers 1061-106n, hereinafter collectively referred to as “carriers 106”).
For example, as a surface charge is produced (e.g., by accumulation or inversion), the effective index of the optical mode decreases due to reduced material index of the silicon in the surface charge region (i.e., the region in which the carriers 106 are produced). Because the thickness of the surface charge region is very thin (e.g., in the range of tens of nanometers), the overlap between the optical mode of the waveguide and the surface charge region is very small, leading to very limited effective index variation of the waveguiding mode. Hence, in order to induce sufficient accumulative phase change, the capacitor 100 must adopt a very long length.
Thus, there is a need for a method and an apparatus for optical modulation.